Experiments and Observations Based on the Finding that Scleractinian Corals Secrete Calcite Instead of Aragonite in Cretaceous Seawater

基于石珊瑚在白垩纪海水中分泌方解石而不是文石的实验和观察

• 批准号: 0447101
• 负责人: Steven Stanley
• 金额: $ 8万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-03-15 至 2007-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0447101&HistoricalAwards=false
• 关键词: Experiments; Observations; Based; Finding; Scleractinian

ABSTRACTThe chemistry of seawater has changed over the past half billion years in response to changes in rates of volcanism in the deep sea. The proposed research relates in particular to changes in the magnesium/calcium ratio of seawater induced by changes in the rate at which hydrothermal activity releases calcium from Earth's mantle into the ocean and consumes magnesium from seawater. Correlations between these changes and the mineralogy of important reef-building and limestone-forming organisms suggest that the magnesium/calcium ratio of the ocean has exerted a strong influence over biomineralization by marine life.At magnesium/calcium ratios below 1, calcium carbonate, the primary constituent of limestone, precipitates inorganically in the form of the mineral low-magnesium calcite. At ratios between 1 and 2, high-magnesium calcite precipitates instead, and at ratios above 2, high-magnesium calcite and/or aragonite (a mineral with a different crystal structure) precipitate. At the present time submarine volcanism is relatively weak, so that the magnesium/calcium ratio of seawater is high (5.2). It can be hypothesized that corals flourish as reef builders today because they secrete aragonite, which is favored by the high ambient magnesium/calcium ratio. During Late Cretaceous time, about 100-65 million years ago, corals yielded their role as dominant reef builders to rudist bivalves, which secreted shells consisting largely of calcite. At that time, the magnesium/calcium ratio of seawater had dropped to the vicinity of 1, favoring calcite precipitation. Corals did not resume their dominant reef-building role until Oligocene time, when the magnesium/calcium ratio was again rising into the aragonite domain.The research now proposed will test the effects of the magnesium/calcium ratio on coral skeletal growth through laboratory experiments. Four species of corals will be cultured in seawaters having 7 different magnesium/calcium ratios, including those of modern and Late Cretaceous seas. Skeletal growth rates and skeletal mineralogies will be compared for the various treatments. Relatively low growth rates in "Cretaceous" seawater would suggest that the low magnesium/calcium ratio of Late Cretaceous seas accounted for the weak reef-building role of corals in those seas. A partial or complete shift in "Cretaceous" seawater to precipitation of calcite instead of aragonite would demonstrate an even stronger influence of seawater chemistry on biomineralization.

在过去的5亿年里，随着深海火山活动速率的变化，海水的化学成分发生了变化。拟议的研究特别涉及由于热液活动将钙从地幔释放到海洋并消耗海水中的镁的速率的变化而引起的海水镁/钙比的变化。这些变化与重要的造礁和石灰岩生物矿物学的相关性表明，海洋的镁/钙比对海洋生物的生物成矿作用产生了强烈的影响。当镁/钙比低于1时，石灰岩的主要成分碳酸钙以矿物低镁方解石的形式无机地沉淀。当比例在1到2之间时，高镁方解石反而析出，当比例超过2时，高镁方解石和/或文石(一种具有不同晶体结构的矿物)析出。目前海底火山作用较弱，海水镁钙比较高(5.2)。可以假设，珊瑚今天作为珊瑚礁建造者蓬勃发展是因为它们分泌文石，而文石是环境中高镁/钙比的有利因素。在大约1亿-6500万年前的白垩纪晚期，珊瑚将其主要的珊瑚礁建造者的角色让给了裸露的双壳类，后者分泌的贝壳主要由方解石组成。当时海水的镁钙比已降至1附近，有利于方解石的沉淀。直到渐新世，当镁/钙比再次上升到文石域时，珊瑚才恢复其主要的造礁作用。现在提出的研究将通过实验室实验来测试镁/钙比对珊瑚骨骼生长的影响。四种珊瑚将在7种不同镁/钙比的海水中养殖，包括现代和晚白垩世海洋的珊瑚。将对不同治疗方法的骨骼生长速度和骨骼矿物学进行比较。“白垩纪”海水中相对较低的生长率表明，晚白垩世海水的低镁钙比是珊瑚在这些海洋中造礁作用较弱的原因。如果“白垩纪”海水部分或全部转变为方解石而不是文石，将证明海水化学对生物矿化的影响更大。